An effective means to increase power generation efficiency of a steam turbine is to raise the turbine steam temperature to 650 deg. C. or more.
Special considerations may be made for these high temperatures. For example, the valve may require cooling features or additionally may be made of high hot strength materials. While cooling means typically have only limited effectiveness, the use of different materials in a valve can introduce problems as different materials have different coefficients of expansion and thus upon heating, unacceptable internal stresses may be created which may adversely affect service life.